Mathematical model and experimental methodology for calibration of a LWIR polarimetric-hyperspectral imager

Abstract

Polarimetric-hyperspectral imaging brings two traditionally independent modalities together to potentially enhance scene characterization capabilities. This could increase confidence in target detection, material identification, and background characterization over traditional hyperspectral imaging. In order to fully exploit the spectro-polarimetric signal, a careful calibration process is required to remove both the radiometric and polarimetric response of the system (gain). In the long-wave infrared, calibration is further complicated by the polarized self-emission of the instrument itself (offset). This paper presents both the mathematical framework and the experimental methodology for the spectro-polarimetric calibration of a long-wave infrared (LWIR) Telops Hyper-Cam which has been modified with a rotatable wire-grid polarizer at the entrance aperture. The mathematical framework is developed using a Mueller matrix approach to model the polarimetric effects of the system, and this is combined with a standard Fourier-transform spectrometer (FTS) radiometric calibration framework. This is done for two cases: one assuming that the instrument polarizer is ideal, and a second method which accounts for a non-ideal instrument polarizer. It is shown that a standard two-point radiometric calibration at each instrument polarizer angle is sufficient to remove the polarimetric bias of the instrument, if the instrument polarizer can be assumed to be ideal. For the non-ideal polarizer case, the system matrix and the Mueller deviation matrix is experimentally determined for the system, and used to quantify how non-ideal the system is. The noise-equivalent spectral radiance and DoLP are also quantified using a wide-area blackbody. Finally, a scene with a variety of features in it is imaged and analyzed.